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Date: Fri, 26 Jan 2024 22:52:57 +0000
From: Justin Stitt <justinstitt@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: linux-hardening@vger.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Bill Wendling <morbo@google.com>, llvm@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/82] overflow: Expand check_add_overflow() for pointer
 addition
Message-ID: <20240126225257.lfgeom4rhjzx2wrf@google.com>
References: <20240122235208.work.748-kees@kernel.org>
 <20240123002814.1396804-1-keescook@chromium.org>
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In-Reply-To: <20240123002814.1396804-1-keescook@chromium.org>

Hi,

On Mon, Jan 22, 2024 at 04:26:36PM -0800, Kees Cook wrote:
> The check_add_overflow() helper is mostly a wrapper around
> __builtin_add_overflow(), but GCC and Clang refuse to operate on pointer
> arguments that would normally be allowed if the addition were open-coded.
>
> For example, we have many places where pointer overflow is tested:
>
> 	struct foo *ptr;
> 	...
> 	/* Check for overflow */
> 	if (ptr + count < ptr) ...
>
> And in order to avoid running into the overflow sanitizers in the
> future, we need to rewrite these "intended" overflow checks:
>
> 	if (check_add_overflow(ptr, count, &result)) ...
>
> Frustratingly the argument type validation for __builtin_add_overflow()
> is done before evaluating __builtin_choose_expr(), so for arguments to
> be valid simultaneously for sizeof(*p) (when p may not be a pointer),
> and __builtin_add_overflow(a, ...) (when a may be a pointer), we must
> introduce wrappers that always produce a specific type (but they are
> only used in the places where the bogus arguments will be ignored).
>
> To test whether a variable is a pointer or not, introduce the __is_ptr()
> helper, which uses __builtin_classify_type() to find arrays and pointers
> (via the new __is_ptr_or_array() helper), and then decays arrays into
> pointers (via the new __decay() helper), to distinguish pointers from
> arrays.
>
> Additionally update the unit tests to cover pointer addition.
>
> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Bill Wendling <morbo@google.com>
> Cc: Justin Stitt <justinstitt@google.com>
> Cc: llvm@lists.linux.dev
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  include/linux/compiler_types.h | 10 +++++
>  include/linux/overflow.h       | 44 ++++++++++++++++++-
>  lib/overflow_kunit.c           | 77 ++++++++++++++++++++++++++++++----
>  3 files changed, 121 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 6f1ca49306d2..d27b58fddfaa 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -375,6 +375,16 @@ struct ftrace_likely_data {
>  /* Are two types/vars the same type (ignoring qualifiers)? */
>  #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
>
> +/* Is variable addressable? */
> +#define __is_ptr_or_array(p)	(__builtin_classify_type(p) == 5)
> +
> +/* Return an array decayed to a pointer. */
> +#define __decay(p)		\
> +	(&*__builtin_choose_expr(__is_ptr_or_array(p), p, NULL))

Initially, I thought this __decay could dereference a NULL which would
be UB.

According to the C std 6.5.3.2 (4):
| The unary * operator denotes indirection. If the operand points to a
| function, the result is a function designator; if it points to an
| object, the result is an lvalue designating the object. If the operand
| has type ‘‘pointer to type’’, the result has type ‘‘type’’. If an
| invalid value has been assigned to the pointer, the behavior of the
| unary * operator is undefined^(84)

With footnote 84 mentioning NULL:
| Among the invalid values for dereferencing a pointer by the unary *
| operator are a null pointer, an address inappropriately aligned for the
| type of object pointed to, and the address of an object after the end of
| its lifetime.

However, in this very same footnote it mentions:
| &*E is equivalent to E (even if E is a null pointer)

So, yeah this is OK ( and new to me :>] )

> +
> +/* Report if variable is a pointer type. */
> +#define __is_ptr(p)		__same_type(p, __decay(p))
> +
>  /*
>   * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
>   *			       non-scalar types unchanged.
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 7b5cf4a5cd19..099f2e559aa8 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -51,6 +51,45 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>  	return unlikely(overflow);
>  }
>
> +/* Always produce an integral variable expression. */
> +#define __filter_integral(x)		\
> +	__builtin_choose_expr(!__is_ptr(x), (x), 0)
> +
> +/* Always produce a pointer value. */
> +#define __filter_ptr(x)			\
> +	__builtin_choose_expr(__is_ptr(x), (x), NULL)
> +
> +/* Always produce a pointer to an integral value. */
> +#define __filter_ptrint(x)		\
> +	__builtin_choose_expr(!__is_ptr(*(x)), x, &(int){ 0 })
> +
> +/**
> + * __check_ptr_add_overflow() - Calculate pointer addition with overflow checking
> + * @a: pointer addend
> + * @b: numeric addend
> + * @d: pointer to store sum
> + *
> + * Returns 0 on success.
> + *
> + * Do not use this function directly, use check_add_overflow() instead.
> + *
> + * *@d holds the results of the attempted addition, but is not considered
> + * "safe for use" on a non-zero return value, which indicates that the
> + * sum has overflowed or been truncated.
> + */
> +#define __check_ptr_add_overflow(a, b, d)		\
> +	({						\
> +		typeof(a) __a = (a);			\
> +		typeof(b) __b = (b);			\
> +		size_t __bytes;				\
> +		bool __overflow;			\
> +							\
> +		/* we want to perform the wrap-around, but retain the result */ \
> +		__overflow = __builtin_mul_overflow(sizeof(*(__a)), __b, &__bytes); \
> +		__builtin_add_overflow((unsigned long)(__a), __bytes, (unsigned long *)(d)) || \
> +		__overflow;				\
> +	})
> +
>  /**
>   * check_add_overflow() - Calculate addition with overflow checking
>   * @a: first addend
> @@ -64,7 +103,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>   * sum has overflowed or been truncated.
>   */
>  #define check_add_overflow(a, b, d)	\
> -	__must_check_overflow(__builtin_add_overflow(a, b, d))
> +	__must_check_overflow(__builtin_choose_expr(__is_ptr(a),	\
> +		__check_ptr_add_overflow(__filter_ptr(a), b, d),	\
> +		__builtin_add_overflow(__filter_integral(a), b,		\
> +				       __filter_ptrint(d))))
>
>  /**
>   * check_sub_overflow() - Calculate subtraction with overflow checking

Does check_sub_overflow() deserve some more love in the future? I
imagine "under"-flowing pointers is not at all common, though.

Nonetheless, this all looks good to me.

Reviewed-by: Justin Stitt <justinstitt@google.com>

> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index c527f6b75789..2d106e880956 100644
> --- a/lib/overflow_kunit.c
> +++ b/lib/overflow_kunit.c
> @@ -45,13 +45,18 @@
>  # define SKIP_64_ON_32(t)	do { } while (0)
>  #endif
>
> -#define DEFINE_TEST_ARRAY_TYPED(t1, t2, t)			\
> -	static const struct test_ ## t1 ## _ ## t2 ## __ ## t {	\
> +#define DEFINE_TEST_ARRAY_NAMED_TYPED(n1, n2, n, t1, t2, t)	\
> +	static const struct test_ ## n1 ## _ ## n2 ## __ ## n {	\
>  		t1 a;						\
>  		t2 b;						\
> -		t sum, diff, prod;				\
> +		t sum;						\
> +		t diff;						\
> +		t prod;						\
>  		bool s_of, d_of, p_of;				\
> -	} t1 ## _ ## t2 ## __ ## t ## _tests[]
> +	} n1 ## _ ## n2 ## __ ## n ## _tests[]
> +
> +#define DEFINE_TEST_ARRAY_TYPED(t1, t2, t)			\
> +	DEFINE_TEST_ARRAY_NAMED_TYPED(t1, t2, t, t1, t2, t)
>
>  #define DEFINE_TEST_ARRAY(t)	DEFINE_TEST_ARRAY_TYPED(t, t, t)
>
> @@ -251,8 +256,10 @@ DEFINE_TEST_ARRAY(s64) = {
>  };
>
>  #define check_one_op(t, fmt, op, sym, a, b, r, of) do {			\
> -	int _a_orig = a, _a_bump = a + 1;				\
> -	int _b_orig = b, _b_bump = b + 1;				\
> +	typeof(a + 0) _a_orig = a;					\
> +	typeof(a + 0) _a_bump = a + 1;					\
> +	typeof(b + 0) _b_orig = b;					\
> +	typeof(b + 0) _b_bump = b + 1;					\
>  	bool _of;							\
>  	t _r;								\
>  									\
> @@ -260,13 +267,13 @@ DEFINE_TEST_ARRAY(s64) = {
>  	KUNIT_EXPECT_EQ_MSG(test, _of, of,				\
>  		"expected "fmt" "sym" "fmt" to%s overflow (type %s)\n",	\
>  		a, b, of ? "" : " not", #t);				\
> -	KUNIT_EXPECT_EQ_MSG(test, _r, r,				\
> +	KUNIT_EXPECT_TRUE_MSG(test, _r == r,				\
>  		"expected "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
>  		a, b, r, _r, #t);					\
>  	/* Check for internal macro side-effects. */			\
>  	_of = check_ ## op ## _overflow(_a_orig++, _b_orig++, &_r);	\
> -	KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, "Unexpected " #op " macro side-effect!\n"); \
> -	KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, "Unexpected " #op " macro side-effect!\n"); \
> +	KUNIT_EXPECT_TRUE_MSG(test, _a_orig == _a_bump, "Unexpected " #op " macro side-effect!\n"); \
> +	KUNIT_EXPECT_TRUE_MSG(test, _b_orig == _b_bump, "Unexpected " #op " macro side-effect!\n"); \
>  } while (0)
>
>  #define DEFINE_TEST_FUNC_TYPED(n, t, fmt)				\
> @@ -333,6 +340,55 @@ DEFINE_TEST_ARRAY_TYPED(int, int, u8) = {
>  };
>  DEFINE_TEST_FUNC_TYPED(int_int__u8, u8, "%d");
>
> +#define DEFINE_TEST_PTR_FUNC_TYPED(n, t, fmt)				\
> +static void do_ptr_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
> +{									\
> +	/* we're only doing single-direction sums, no product or division */ \
> +	check_one_op(t, fmt, add, "+", p->a, p->b, p->sum, p->s_of);\
> +}									\
> +									\
> +static void n ## _overflow_test(struct kunit *test) {			\
> +	unsigned i;							\
> +									\
> +	for (i = 0; i < ARRAY_SIZE(n ## _tests); ++i)			\
> +		do_ptr_test_ ## n(test, &n ## _tests[i]);		\
> +	kunit_info(test, "%zu %s arithmetic tests finished\n",		\
> +		ARRAY_SIZE(n ## _tests), #n);				\
> +}
> +
> +DEFINE_TEST_ARRAY_NAMED_TYPED(void, int, void, void *, int, void *) = {
> +	{NULL, 0, NULL, NULL, NULL, false, false, false},
> +	{(void *)0x30, 0x10, (void *)0x40, NULL, NULL, false, false, false},
> +	{(void *)ULONG_MAX, 0, (void *)ULONG_MAX, NULL, NULL, false, false, false},
> +	{(void *)ULONG_MAX, 1, NULL, NULL, NULL, true, false, false},
> +	{(void *)ULONG_MAX, INT_MAX, (void *)(INT_MAX - 1), NULL, NULL, true, false, false},
> +};
> +DEFINE_TEST_PTR_FUNC_TYPED(void_int__void, void *, "%lx");
> +
> +struct _sized {
> +	int a;
> +	char b;
> +};
> +
> +DEFINE_TEST_ARRAY_NAMED_TYPED(sized, int, sized, struct _sized *, int, struct _sized *) = {
> +	{NULL, 0, NULL, NULL, NULL, false, false, false},
> +	{NULL, 1, (struct _sized *)(sizeof(struct _sized)), NULL, NULL, false, false, false},
> +	{NULL, 0x10, (struct _sized *)(sizeof(struct _sized) * 0x10), NULL, NULL, false, false, false},
> +	{(void *)(ULONG_MAX - sizeof(struct _sized)), 1, (struct _sized *)ULONG_MAX, NULL, NULL, false, false, false},
> +	{(void *)(ULONG_MAX - sizeof(struct _sized) + 1), 1, NULL, NULL, NULL, true, false, false},
> +	{(void *)(ULONG_MAX - sizeof(struct _sized) + 1), 2, (struct _sized *)(sizeof(struct _sized)), NULL, NULL, true, false, false},
> +	{(void *)(ULONG_MAX - sizeof(struct _sized) + 1), 3, (struct _sized *)(sizeof(struct _sized) * 2), NULL, NULL, true, false, false},
> +};
> +DEFINE_TEST_PTR_FUNC_TYPED(sized_int__sized, struct _sized *, "%lx");
> +
> +DEFINE_TEST_ARRAY_NAMED_TYPED(sized, size_t, sized, struct _sized *, size_t, struct _sized *) = {
> +	{NULL, 0, NULL, NULL, NULL, false, false, false},
> +	{NULL, 1, (struct _sized *)(sizeof(struct _sized)), NULL, NULL, false, false, false},
> +	{NULL, 0x10, (struct _sized *)(sizeof(struct _sized) * 0x10), NULL, NULL, false, false, false},
> +	{NULL, SIZE_MAX - 10, (struct _sized *)18446744073709551528UL, NULL, NULL, true, false, false},
> +};
> +DEFINE_TEST_PTR_FUNC_TYPED(sized_size_t__sized, struct _sized *, "%zu");
> +
>  /* Args are: value, shift, type, expected result, overflow expected */
>  #define TEST_ONE_SHIFT(a, s, t, expect, of)	do {			\
>  	typeof(a) __a = (a);						\
> @@ -1122,6 +1178,9 @@ static struct kunit_case overflow_test_cases[] = {
>  	KUNIT_CASE(s32_s32__s32_overflow_test),
>  	KUNIT_CASE(u64_u64__u64_overflow_test),
>  	KUNIT_CASE(s64_s64__s64_overflow_test),
> +	KUNIT_CASE(void_int__void_overflow_test),
> +	KUNIT_CASE(sized_int__sized_overflow_test),
> +	KUNIT_CASE(sized_size_t__sized_overflow_test),
>  	KUNIT_CASE(u32_u32__int_overflow_test),
>  	KUNIT_CASE(u32_u32__u8_overflow_test),
>  	KUNIT_CASE(u8_u8__int_overflow_test),
> --
> 2.34.1
>
Thanks
Justin